1. Technical Field
The present disclosure relates to multi-color printing systems, and, in particular, to a system and method for characterizing color separation misregistration of a multi-color printing system utilizing a multi-channel scanner.
2. Description of Related Art
In multi-color printing systems a limited number of color separations are used for marking a substrate for achieving a wider variety of colors, with each separation marking the substrate using discrete shapes, such as dots having a circular or oval shape, or periodic line patterns. This concept is generally known as color halftoning, and involves combining two or more patterned separations on the substrate. The selection of color separations and halftone pattern designs are carefully chosen for achieving a visual effect of the desired color.
Many prior art printing systems use cyan, magenta, yellow and black (also referred to as CMYK) color separations that mark a substrate using discrete cluster dots. The dots may be marked in a dot-on-dot fashion, by marking the substrate with a first and second color separation, with the dots of the second color separation superimposed over the dots of the first color separation for achieving the desired color. In addition, the dots may be applied in a dot-off-dot fashion, with the dots of the second color separation placed in the voids of the dots of the first color separation for achieving the desired color. However, multi-color printing systems are susceptible to misregistration between color separations due to a variety of mechanical related issues. For both dot-on-dot and dot-off-dot rendering, color separation misregistration may cause a significant color shift in the actual printed color that is noticeable to the human eye.
Broadband multi-channel scanners are widely available. Typically, they include a plurality of channels each of which are responsive to a wide spectrum of optical wavelengths. Since the human eye has three types of daytime optical receptors (i.e., cone cells), broadband multi-channel scanners usually contain 3 channels, each of which are usually referred to as “Red”, “Blue” and “Green” channels. Therefore, these broadband three-color scanners are called “RGB” scanners.
A widely used marking technology includes using rotated cluster dot sets since anomalies (e.g., color shifts) due to color separation misregistrations are subtle and less detectable by the human eye. However, even in these cases color misregistrations can be objectionable, particularly at edges of objects that contain more than one separation. Therefore, it is important to characterize color separation misregistration in order to perform corrective action in the print engine.
Many other methods for characterizing misregistration of color separations include using physical registration marks. The registration marks include two fine straight lines, each line formed using a different color separation. The two lines are aligned and joined to form one straight line. Alignment of the two lines is analyzed, with misalignment indicating misregistration of one of the color separations relative to the other. The analysis may include studying the printed registration marks with a microscope and visually determining if misregistration has occurred. Such analysis is tedious and not conducive to automation. The analysis may include imaging the marker with a high resolution scanning device and analyzing the high resolution scanned image using complex software for determining the positions of the registration marks relative to one another. These types of analysis sometimes require high-resolution scanning equipment and may involve a significant amount of computational power.
In another method used for higher end printer devices outputting high volume and/or high quality images, misregistration of color separations is characterized by measuring the transition time between the edges of two primary separation patches (e.g., cyan and magenta) on a moving photoreceptor belt. The patches have angled edges (e.g., chevrons) that allow the determination of misregistration in both the fast scan direction (transverse to the longitudinal axis of the photoreceptor belt) and slow scan direction (parallel to the longitudinal axis of the photoreceptor belt). Simple photo detectors are used to measure the time between the moving edges of the chevrons, and this can in turn be used to compute the misregistration in both slow and fast scan directions. However, there is a continuing need to characterize color separation misregistration effectively and/or efficiently.